Re: Handling void*

From:
red floyd <no.spam.here@example.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Fri, 7 Nov 2008 09:28:59 CST
Message-ID:
<nSQQk.4285$Ei5.1121@flpi143.ffdc.sbc.com>
Seungbeom Kim wrote:

Martin T. wrote:

Thomas Maeder wrote:

No. There is not even a guarantee that converting from function
pointer to void * and later converting back to the function pointer
type yields a valid function pointer.


Hu?
I thought that's about the only thing guaranteed with void* pointers,
namely that I can cast them back to the original type??

int i;
void * pvi = &i; // OK
int* pi = static_cast<int*>(pvi); // OK (?)


That's true only for pointers to objects, not for pointers to functions
(3.9.2/4, 4.10/2, 5.2.9/10).

Pointers to functions can be converted only to pointers to functions
of other types by reinterpret_cast, and converted back to the original
type (5.2.10/6).


This is a hole in the type system. There is no portable way to display
the value of a function pointer; I.e. there is no

ostream& operator<<(ostream&, void(*)());

So, if, for debugging purposes, I want to dump a pointer to function, I
have to rely on inherently nonportable stuff, or else do something like
the following, which is byte-order dependent.

ostream& operator<<(ostream& os, void(*f)())
{

     const unsigned char *p = reinterpret_cast<unsigned char *>(&f));
     for (size_t j = 0 ; j < sizeof(f) ; ++j)
        os << hex << *p++;
}

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